The present invention relates to an apparatus for calling a central computer and transmitting metering information over the telephone lines. In particular the present invention relates to a dial inbound Meter Interface Unit ("MIU") which derives its power from the telephone line, particularly from a ringing telephone line, to which the MIU is connected.
As used herein, the term Meter Interface Unit ("MIU") refers to an electronic unit located at the residence or business location of a customer of a public utility (water, gas, electricity, or heat). While the term "MIU" is generally used to refer to devices which interface a utility meter with a medium such as telephone or radio ("RF"), as used herein, the term "MIU" will be limited to telephone MIUs, particularly telephone MIUs which are used for dial inbound purposes.
A dial inbound MIU is one which calls a central computer system located at an office of the public utility. The central computer is typically equipped with a number of modems and telephone lines for placing and receiving telephone calls to and from remote MIUs which are located at customers' locations. A dial inbound MIU is an MIU which is used to call the central computer from a customer's location.
Heretofore, dial inbound MIUs have been used to dial central computers for the purpose of transmitting utility metering data from the customers' meters to the utility. The MIU dials the central computer periodically or at a time which has been predetermined and entered into the MIU, either directly (typically at the time the MIU is first installed at a customer's location) or downloaded to the MIU by the central computer (when the MIU dials the central computer to upload metering information). In a typical transaction, an MIU reads metering information from a meter, dials the central computer, and uploads the metering data via modem to the central computer. While the MIU is connected to the central computer, the central computer may download certain information to the MIU, such as the time at which the MIU should next send meter data to the central computer. Thus, one feature of the MIU is that it must have knowledge of its next call back time. In order to retain this information, the MIU typically has a semiconductor device called a real time clock which retains either the real time, typically expressed as month, day, year and hour, minute, second, or, alternatively, a countdown of days, hours, minutes, and seconds (although possibly expressed as only hours, minutes, seconds, or even only as seconds) to the next call.
In a typical installation at a customer's residence, there may be only a single telephone line which serves the residence. Accordingly, there must be provisions within the MIU to enable the MIU to detect whether the telephone line is in use, so that the MIU can determine whether it can make use of (seize) the telephone line. In addition, after the MIU has seized the telephone line, it must be able to detect whether an extension to the seized telephone line has been lifted off-hook by someone at the location, in which event the MIU must release the telephone line to the other user.
In view of the electronic circuitry within an MIU and the typical desire to prevent power outages from causing the circuitry to lose information, such as the current time or call-back time, both of which require the real time clock circuit, it is desirable for an MIU to be able to be operated independently of the AC power at the customer's location. Thus, it has heretofore been typical for dial inbound MIUs to be operated by either internal or external batteries.
U.S. Pat. Nos. 4,455,453; 4,469,917; 4,847,892; and 4,578,534 all disclose dial inbound MIUs which are powered by external batteries. The use of an external battery renders an MIU more costly to maintain than an internal battery. In addition, external battery supplies can interfere with the installation of MIUs in confined spaces, such as pits or service boxes.
U.S. Pat. No. 4,644,103 discloses an MIU which derives its operating current from a telephone line. As described in that patent, telephone line current is used to energize a tone detection circuit in the MIU. The tone detection circuit is responsible for determining if the MIU is being interrogated by the central computer. The tone detection circuit operates intermittently. Power is used every few seconds to ensure that the circuit is functioning properly. The short powered discharge intervals associated with the device limit its capability solely to signal detection. In addition, the telephone must be frequently placed in the off-hook state to accommodate recharging of a voltage regulating capacitor. Such constant recharging of the capacitor increases the likelihood that there will be interference with the customer's use of the telephone line.
A legal limitation on the use of the telephone line as a source for powering an MIU is found in the Code of Federal Regulations, 47 C.F.R. .sctn.68.312, which limits the amount of current which can be drawn from a telephone line while the telephone is in the "on-hook" state. In the on-hook state, the telephone handset is placed on the telephone receiver, where it depresses a switch which disconnects the telephone instrument from the telephone line, resulting in an open circuit. When a telephone instrument is in the on-hook state, it is commonly referred to as being "hung up". While in the on-hook state, a device connected to the telephone line must have an effective DC resistance between the pair of telephone wires (which are called "tip" and "ring") of the telephone line which is greater than 5 megohms for all DC voltages up to and including 100 volts. A 5 megohm resistance would correspond to a draw of about 9.6 microamps if the voltage between the tip and ring wires was about 48 volts.
When the handset is lifted "off-hook", the telephone is connected to the line, and current flows through the telephone instrument. In the off-hook state, the telephone is available for use by the customer (or by a device connected to the telephone lines). In accordance with the Code of Federal Regulations, 47 C.F.R. .sctn.68.312, while off-hook, there is no limit with respect to the amount of DC current that may be drawn from the telephone line.
In U.S. Pat. No. 5,454,031 ("the '031 patent"), a dial inbound MIU is described which derives its power from a telephone line, storing the power in a double layer capacitor. As described in that patent, DC current can be drawn from the telephone line, while the telephone is in the on-hook state, to charge the double layer capacitor. Further, that patent describes drawing additional DC current from the telephone line while the telephone is in the off-hook state for charging the double layer capacitor. As described in the '031 patent, when the telephone line is on-hook DC current is drawn through a large resistor (10 megohm). As the specifications for a telephone line requires only a minimum of 21 volts between tip and ring when the telephone is in the on-hook state, the amount of DC current which can be drawn by the circuit described in that patent is limited to as little as 2.1 microamps. While the '031 patent describes the charging of the double layer capacitor using the DC current which is available while the telephone is both in the on-hook state and when the telephone is in the off-hook state, it specifically fails to mention one particularly useful charging opportunity. In particular, 47 C.F.R. .sctn.68.312 permits a telephone device to draw considerably more current when on-hook and ringing, than when on-hook and not ringing, as during a ringing condition, AC current is available.